This application claims the priority benefit of Taiwan application Ser. No. 91100278, filed Jan. 11, 2002.
1. Field of Invention
The present invention relates to a control technology and apparatus for bias correction on an optical modulator of optical emitter of external modulation type. More particularly, the present invention relates to a control method and apparatus used for a bias controller of automatic bias correction on an optical modulator of optical emitter of external modulation type.
2. Description of Related Art
In recent years, the optical fiber communication in the world has been well and greatly developed in flourish, wherein the internet and the cable TV have astonishing development. Under attraction of great business chance in the market with a large number of users, all dealers of the optical fiber communication have been developing for the goals of longer distance in fiber communication, wider bandwidth, and better quality. In FIG. 1, since the optical-fiber emitter 110 with a wavelength of 1559 nm can have low distortion and low insertion loss, and can implement with an optical amplifier 120, an optical receiver 130, so as to have the long distance network transmission with high quality. Therefore, it is greatly pushed and made to wide applications. In FIG. 2A and FIG. 2B, the optical-fiber emitter can also be divided into a direct modulation type optical emitter and an external-modulation type optical emitter as shown in FIGS. 2A and 2B. The main difference of them is that the direct-modulation type optical emitter 240 is directly adding the transmission signal RF to the laser source 210, whereby an optical form is exported through the optical amplifier. Also and, the external-modulation type optical emitter 280 is that when the output of the laser source 250 is input to the optical modulator 260, the transmission signal 250 is externally added to the optical modulator 260. Then, the optical amplifier 270 amplifies the signal and exports the optical output. The characteristic curve of the optical modulator is shown in FIG. 3. In FIG. 3, it is observed that the characteristic of the optical modulation can be changed by using the external bias. In general, if it is applied on the high linear transmission system, such as the analog signals of the cable TV, the optical modulation bias can be adjusted to be located at the Quad point, so as to obtain the second order linearity with less distortion. If it is applied on the digital transmission system, the bias can be set at the peak or the null.
However when it is in the practical application, a phenomenon is found that the bias point of the optical modulator used in a high linear transmission system will shift along with the environmental temperature. If the bias point of the optical modulator has an error greater than +/xe2x88x921 degree, it would cause the distortion of the second order non-linearity to be severely degraded. Therefore, for an optical emitter with external modulation type in commercial use, in order to obtain the high linear transmission with low distortion, it is necessary to set the bias voltage of the optical modulator to the bias point Quad. Also and, an offset is made on the shift of bias point Quad, which is caused by the temperature.
Currently, many solutions, such as the U.S. Pat. No. 5,812,297 and U.S. Pat. No. 5,343,324, are proposed on the bias voltage control of the optical modulator. FIG. 4 is a conventional bias voltage controller of optical modulator of optical emitter for a external modulation type. The conventional bias voltage controller uses a 10.85 MHz signal generator 410 to generate a 10.85 MHz base signal, which is combined with a DC voltage and then sent to a bias voltage input terminal of the optical modulator. When a shift occurs at the best bias point of the optical modulator, that is the Quad point in FIG. 3, it will cause a distortion of the second order non-linearity in the transmission signal transmitted by the optical modulator. Then, a signal of 21.7 MHz is generated from the transmission signal exported by the optical modulator. This transmission signal goes through an optical coupler with 90:10 splitting and enters an optical receiver 420, so as to convert the transmission signal into an electrical signal. The electrical signal is input to a wave detector 430. The wave detector 430 can be a multiplying wave detector, a diode wave detector, an integrated circuit, or a discrete circuit. The wave detector 430 generates an error voltage, according to an amplitude and a phase of the signal of 21.7 MHz. The error voltage could be very small, and is necessary to be amplified by a DC-level amplifier 450 after passing the narrow-frequency low-pass filter 440. In order to filter out the noise after amplification, it has to pass another narrow-frequency low-pass filter 460. The error voltage is then enters a DC level shifter 470 for shifting the error voltage to a level, which is acceptable by the microprocessor 490. An analog-to-digital converter (ADC) 480 further converts the error voltage and exports the output to the microprocessor 490. According to the quantity of the error voltage, the microprocessor 490 controls a digital-to analog converter (DAC) 495 to export an offset voltage, so as to correct the bias voltage point of the optical modulator to the Quad point.
However, both the wave detector and the DC amplifier themselves have characteristic of producing compensation effect and zero-point shifting effect. In this situation, when the temperature changes, they will produce an extra voltage to the output of the bias voltage controller. This voltage would cause that the bias voltage controller cannot provide a proper offset voltage for correcting the bias voltage of the optical modulator to remain at the Quad point.
The invention provides a control method and an apparatus used for a bias controller of automatic bias correction on an optical modulator of external-modulation optical emitter. The correct offset voltage can be automatically measured and the correction is automatically made, so that the thermal effect can significantly reduced, and the output voltage can be adjusted.
The invention provides a control method for automatic bias correction on an optical modulator of external-modulation optical emitter. The bias controller provides a base signal, which is also combined with a DC voltage to form a bias voltage control signal. When the bias voltage control signal is input to the optical modulator, the bias voltage controller also receives the output of a first second-order harmonic component signal from the optical modulator. The bias voltage controller also takes a second second-order harmonic component signal generated from a base signal generator, which has the same frequency as the first second-order harmonic component signal. The bias voltage controller compares the first second-order harmonic component signal with the second second-order harmonic component signal, so as to produce a DC voltage for controlling the bias voltage of the optical modulator. The automatic correcting method includes dividing the method into a measuring cycle and a correcting cycle. When the bias voltage controller is at the measuring cycle, the receiving terminal is grounded. Then, the extra offset voltage and the zero shifting voltage from the bias voltage controller are measured. When the bias voltage controller is at the correcting cycle, the receiving terminal receives the first second-order harmonic component signal, and then compares the first second-order harmonic component signal with the second second-order harmonic component signal, so as to obtain a DC error voltage. The DC error voltage is subtracted from the extra offset voltage and the zero shifting voltage, so as to obtain a corrected DC error voltage. The corrected DC voltage is used to control the bias voltage of the optical modulator.
The invention provides a correcting apparatus for automatically correcting an optical modulator of external-modulation optical emitter. When the correcting apparatus issues a bias voltage control signal to the optical modulator, the correcting apparatus also receives an output from the optical modulator about a first second-order harmonic component signal. According to the first second-order harmonic component signal, the correcting apparatus produces a DC voltage to control a bias voltage of the optical modulator. The correcting apparatus comprises a base signal generator, a combiner, an optical coupler device, an optical receiver, a resistor, a RF switch, a wave detector, a first narrow-band low pas filter (LPF), a DC amplifier, a second narrow-band LPF, a DC level shifter, an ADC, microprocessor, and a DAC. The base (tone) signal generator generates a base signal and produces a second second-order harmonic component signal based on the base signal. The combiner is coupled to the base signal generator to receive the base signal and the DC voltage, and export the bias voltage control signal to the optical modulator. The optical coupler receives and exports the first second-order harmonic component signal. The optical receiver receives the first second-order harmonic component signal from the optical coupler and exports it. The resistor has a first terminal and a second terminal, where the second terminal is connected to a ground voltage. The RF switch is used to select for receiving the first second-order harmonic component signal from the optical receiver or connecting to the resistor. The wave detector is coupled to the base signal generator and the RF switch, so as to export the first second-order harmonic component signal, the second second-order harmonic component signal, and a DC error voltage signal. The first narrow-band LPF is coupled to the wave detector. The DC amplifier is coupled to the first narrow-band LPF. The second narrow-band LPF is coupled to the DC amplifier. The DC level shifter is coupled to the second narrow-band LPF. The ADC is coupled to the second narrow-band LPF. The microprocessor is coupled to the ADC and the RF switch. The DAC is coupled to the microprocessor and the combiner, so as to export the DC voltage.
In summary, the invention employs the microprocessor to control the RF switch, so as to obtain the extra offset voltage and the zero shifting voltage induced by the loop circuit of the bias voltage controller. According to these two voltages, the actual error voltage is corrected, whereby the bias of the optical modulator can be set to the correct bias voltage point. Additionally, a measuring cycle and a correcting cycle are repeated, so as to automatically correct the bias voltage controller. As a result, the temperature effect on the bias voltage shifting can be effectively reduced, and the voltage output of the bias voltage controller can be modulated.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.